The invention concerns a liquid-cooled piston.
The pistons of internal combustion engines are subjected to great thermic stress due to combustion which occurs in the combustion chamber. For this reason, it is advisable, especially in diesel engines and supercharged engines, to ensure the cooling of pistons by introducing a coolant into cavities in the piston.
A liquid cooled piston is known from DE-OS 30 19 953. This piston has an annular duct, to which an open bore which extends to the crank space is adjoined, through which the cooling oil can flow in. Oil flows out of the known piston through an outlet bore, which is located at approximately the center of the piston (when seen from above). It is also possible to position the coolant outflow in a position which is diametrically opposite the inflow. In the known piston, the entire annular duct is located at a specific height of the piston. Hereby, a useful cooling of the fire land area, i.e. the area behind the piston ring, as well as the area below the combustion chamber is achieved. The piston-pin bores, which are subject to great stress particularly in high speed diesel engines, and the area surrounding these, are not however sufficiently cooled.
This disadvantage is also true of the piston according to DE 195 22 756 A1, in which cooling of the area behind the piston rings is achieved by bores which extend substantially in the direction of the piston axis.
Also, in pistons according to DE 34 44 661 A1, the cooling ducts, which are arranged in a star-shape, are so formed that they cool in particular the area behind the rings as well as the combustion chamber area. Satisfactory cooling of the piston-pin boss area can only be ensured by providing complicated casting cores which can only be removed from the completed pistons with great effort.
The cooling ducts of the piston according to DE 196 18 625 C1 are comparatively simply constructed, however they are located so far away from the piston-pin boss area that adequate cooling is not achieved in this zone.
A piston having an annular cooling duct, the top-side of which has asymmetric ramps and the bottom-side has troughs offset in a circumferential direction is known from DE 198 10 937 C1, which was not pre-published. Efficient delivery of the oil through the cooling ducts is hereby supposed to be achieved.
German Patent DE 197 36 135 C1 discloses a liquid-cooled piston having a cooling duct which, with two separate limbs, extends upwardly inclined starting from a radial edge to the center and thereafter extends downwardly inclined to the opposite radial edge. When seen from above, the two limbs as a whole form an X-shape. This should lead to improved cooling of the piston head.
Finally a piston for internal combustion engines with inclined or horizontally located cylinders is known from DE PS 17 51 342, where steps which are offset against each other are located on the inner walls of the cooling duct to achieve a reliable delivery and flow of the cooling duct also in inclined pistons. However, the cooling duct is also too far away from the piston-pin boss area in this case to ensure sufficient cooling in this zone in modern, high speed diesel engines. Furthermore, the complex internal design of this cooling duct requires complex casting cores.
The object of the invention is to create a liquid-cooled piston which is easy to produce and which effectively cools the ring area as well as the piston-pin boss area, wherein the strength demands placed on the piston continue to be met.
As a result, the cooling duct of the piston according to the invention which, when viewed as an entirety from above, has an extensive annular shape or consists of several annular segments which are designed in the direction of the axis piston in an undulated manner. Herein, the cross section of the cooling duct remains basically the same across the whole extent of the cooling duct, so that for example in contrast to the piston according to DE-PS 17 51 342, no unnecessary complication of the cooling duct design is required.
Rather the cooling duct extends with a substantially constant cross section in an undulated manner as seen in a side-view of the piston, so that it extends from the area behind the piston rings closer to the piston-pin boss in certain sections than is the case with known pistons. A reliable cooling of both these areas can thus be achieved. Furthermore, the undulated design presents a further advantage in that the cooling duct will be longer in total so that in contrast to conventional cooling ducts, it has a larger cooling surface, and cooling performance can be increased. In addition, the undulated shape of the cooling duct allows for a smaller distance between the cooling duct and the annular support, since sufficient material remains behind the annular supports in each of the wave troughs and thus strength demands will be met as a whole.
The undulated shape of the cooling duct also ensures that, in contrast to known cooling ducts which remain at one level, the cooling oil does not flow straight through, but rather remains in the cooling duct for a longer dwell time and can thus absorb more heat. Finally, in contrast to conventional cooling ducts, cooling of the areas surrounding the cooling duct does not only occur on one side. To a certain extent, the cooling duct not only cools the areas surrounding it, but with regard to a wave trough, cooling also occurs from the wave trough as well as from both neighboring wave crests in the direction of the wave trough. This mode of action can also improve the cooling performance.
In tests, one design of the cooling duct, in which the distance between the wave trough and the wave crest was 1.5 times greater than the cross section of the cooling duct, proved to be particularly advantageous. In other words the measured distance extending in the direction of the axis piston between the lowest point of a wave trough and the highest point of a wave crest is at least 1.5 times the distance between the lowest point of a wave trough and the highest point of the cooling duct in the wave trough, which corresponds to the cross section of the cooling duct.
The measures according to the invention advantageously allow the thickness of the wall between the cooling duct and the annular supports to be reduced to 0-10 mm, preferably 0-5 mm, and in particular 0-2 mm. To improve cooling, the cooling duct can hereby run particularly close to the annular supports, and even have contact with them so that the inside surface of the annular support intermittently touches the cooling duct. A wall thickness which is sufficient to provide adequate stability remains herein due to the undulated progression in the wave troughs.
The design of the undulated cooling duct should preferably have an uneven number of complete waves in one half of the annular extension of the cooling duct. In other words, an uneven number of wave crests can be seen in an imaginary, sectional side view through the cooling duct so that a wave crest is located above the piston-pin bore. If there is an even number, a wave trough would be located here so that in certain situations there would be an insufficient material thickness between the wave trough and the piston-pin bore. Such an undesirable weakness in the piston can be reliably avoided if the number of waves is uneven so that a wave crest is located above the piston-pin bore, which in view of its shape fits particularly well to the form of the upper half of the piston-pin bore.
Finally, with regard to the shape of the cross section of the cooling duct, an oval design, which is preferably inclined outwards, has proved to be particularly advantageous. In other words, in a sectional side view, the upper section of the oval shaped cooling duct is closer to the outer wall of the piston than the lower section.